Cardiac arrhythmias (e.g., fibrillation) are irregularities in the normal beating pattern of the heart and can manifest themselves in either the atria or the ventricles of the heart. For example, atrial fibrillation is a form of arrhythmia characterized by rapid randomized contractions of atrial myocardium, causing an irregular, often rapid ventricular rate. The regular pumping function of the atria is replaced by a disorganized, ineffective quivering as a result of chaotic conduction of electrical signals through the upper chambers of the heart. Atrial fibrillation is often associated with other forms of cardiovascular disease, including congestive heart failure, rheumatic heart disease, coronary artery disease, left ventricular hypertrophy, cardiomyopathy, or hypertension.
It is now understood that recurrent atrial fibrillation (paroxysmal and persistent) is triggered by rapidly firing tissue, (called “ectopic foci”), that are located in one or more of the four pulmonary veins, which attach to the rear of the left atrium. It has been found that atrial fibrillation may be cured by electrically isolating the pulmonary veins from the rest of the atrium.
Various techniques have been employed for pulmonary vein isolation. A common purpose of each of these techniques is to replace cardiac muscle cells with scar tissue, which scar tissue cannot conduct normal electrical activity within the heart.
In one known approach, circumferential ablation of tissue within the pulmonary veins has been practiced to treat atrial fibrillation. By ablating the heart tissue at selected locations, electrical conductivity from one segment to another can be blocked and the resulting segments become too small to sustain the fibrillatory process on their own.
Several types of ablation devices have recently been proposed for creating lesions to treat cardiac arrhythmias. Many of the recently proposed ablation instruments are percutaneous devices that are designed to create such lesions from within the heart. Such devices are positioned in the heart by catheterization of the patient, e.g., by passing the ablation instrument into the heart via a blood vessel, such as the femoral vein.
Typically, percutaneous devices are positioned with the assistance of a guide wire, which is first advanced into heart. In one common approach, a guide wire or similar guide device is advanced through the left atrium of the heart and into a pulmonary vein. A catheter instrument with an expandable element is then advanced over the guide wire and into the pulmonary vein where the expandable element (e.g., a balloon) is inflated. The balloon includes a circumferential ablation element, e.g., an energy emitting device, such as a laser, disposed in the inner surface of the balloon, which performs the ablation procedure.
It is noted that ablation within the pulmonary vein can result in complications. Overtreatment deep within a vein can result in stenosis (closure of the vein itself), necrosis or other structural damage, any of which can necessitate immediate open chest surgery. Conversely, undertreatment in which scar tissue formed is not continuous and/or insufficient to replace the cardiac muscles sought to be electrically isolated from the atrium will cause the surgical ablation procedure to be unsuccessful. Thus, repeating of the surgical ablation procedure is then required which is almost always undesirable. The term “continuous” in the context of a lesion is intended to mean a lesion that substantially blocks electrical conduction between tissue segments on opposite sides of the lesion.